The present invention relates to a system provided in a CATV (cable television) for producing a jamming signal, thereby preventing nonsubscribers from poaching pay television programs.
A subscriber who contracted with a center for CATV must pay a fee monthly or a fee for every program in order to receive television programs from the CATV. Accordingly, it is necessary to be able to prevent the poaching of pay television programs. There are some scrambling systems where the television signal is processed at the center so that the television signal can not be received through an ordinary television set.
In a jamming method as one of the scrambling systems, an interference wave or noise is inserted in a band of the CATV signal.
FIG. 13 shows a conventional jamming signal producing system. The system is provided in a trunk line to subscribers.
A TV signal from a center is applied to a distributor 11a of the system through a buffer amplifier 11. The signal is distributed and applied to individual television sets of subscribers through combiners 10, 10a and 10b, respectively.
The jamming signal producing system comprises a CPU 1 having a memory 5, a PLL (phase-locked loop) including a phase detector 2, a 1/N divider 2a and a VCO (voltage-controlled oscillator) 3, an A/D converter 4 and a D/A converter 6. The CPU 1 is operatively connected to the PLL 2, A/D converter 4, and D/A converter 6. The phase detector 2 is connected to the VCO 3 through a switch S1 and to the A/D converter 4. The switch S1 has two fixed contacts C1 and C2 operated by an output of the CPU 1. The D/A converter 6 is connected to the VCO 3 through the contact C2 of the switch S1. The VCO 3 is connected to switches S2, S2a and S2b through a buffer amplifier 7. The switches S2, S2a and S2b are closed by the output of the CPU 1 to connect VCO 3 to the low pass filters 9, 9a and 9b, respectively.
To generate a jamming signal, the CPU 1 operates to close the contact C2 of the switch S1. The data stored in the memory 5 is applied to the D/A converter 6 in which the data is converted into an analog signal. The analog signal is applied to the VCO 3 through the switch S1. The VCO 3 oscillates, namely produces a jamming signal having a frequency based on the data. The jamming signal is applied to the combiners 10, 10a and 10b through the low pass filters 9, 9a and 9b accordance with the operation of the switches S2, S2a and S2b. The jamming signal is combined with the TV signal from the distributor 11a in the combiners 10, 10a and 10b and the combined signals are fed to individual television sets of the subscribers.
As shown in FIG. 14, in the combined signal, the jamming signal Fj is inserted a band including a video signal FV and an audio signal Fa. The frequency of the jamming signal stands from the video signal Fv by 250 kHz.
If temperature or humidity around the jamming signal producing device changes, a drift occurs in the frequency of the jamming signal. If the drift occurs, the frequency interval of 250 kHz is not maintained so that a sufficient scramble effect is not obtained, or the jamming signal interferes an adjacent channel. Thus, it is necessary to perform calibration to correct the drift.
In calibration operation, the CPU 1 operates to close the contact C1 of the switch S1. The output of the phase detector 2 is applied to the A/D converter 4 wherein the output signal is converted into a digital signal. The digital signal is applied to the CPU 1 and compared with the data stored in the memory 5, so that an output signal in consideration of the drift is produced from the CPU 1 and applied to the D/A converter 6. Then, the contact C2 closes, and hence the analog signal converted at the O/A converter S is applied to the VCO 3 which oscillates the jamming signal at a calibrated frequency. The calibration is continuously performed at interval of 10 to 15 minutes.
In order to effectively scramble the television signal, it is necessary to accurately control the amplitude, frequency and cycle of the jamming signal.
It is recognized by experience that the amplitude of the jamming signal is selected to increase the jamming signal by 12 to 14 dB, the frequency is larger than the video signal Fv by 250 kHz, and the cycle for jamming the image is several tens msec.
In the system, since the drift is often calibrated, the jamming signal is not sufficiently inserted the TV signal, and it is impossible to scan the channel to be scrambled at high speed so that the scramble effect can not be ensured.
Further, the reduction of the lock-up time in the PLL circuit is not considered, which also aggravates the scramble effect.
The A/D and D/A converters provided in the system are expensive, causing the increase of the manufacturing cost of the system.